1. Field of the Invention
This invention relates to equipment designed for "in-situ" cleaning of low pressure chemical vapor deposition (LPCVD) furnace quartzware at furnace operating temperatures.
2. Description of the Prior Art
As integrated circuits continue to increase in complexity, films formed by LPCVD occur frequently in the fabrication process. Many of these films are deposited in a furnace type of LPCVD system. While such systems produce uniform films very economically, the "hotwall" system suffers from a film buildup on the furnace quartzware. The film can break loose during the process causing unacceptably high levels of particulate contamination non the wafer surface. To overcome this problem frequent cleaning of furnace quartzware is necessary to achieve and maintain high semiconductor product yields.
To maintain the required levels of cleanliness in furnace LPCVD systems, current practice involves physically removing the quartz tube and wafer boat, allowing them to cool, soaking them in an acid bath, rinsing them with DI water, drying them, reinstallation, thermal stabilization, leak checking and process requalification. The whole process can take several hours during which time the system is inoperable for wafer production.
Alternatively, the "dirty tube" is replaced by a previously cleaned tube but two to three hours are still involved in reestablishing the furnace temperature profile, leak checking the sytem and performing a test run to ensure that tube changing has not altered film deposition conditions.
While practice varies, depending on company experience and product complexity, the cleaning process is typically performed after a 5 micron film buildup, which occurs after 10 to 20 deposition hours of polysilicon and silicon nitride, respectively. With a conservative "inoperative time" estimate of 2 hours (and it is often much longer), this translates into a 10% to 20% loss of useful processing time.
In addition to this "productivity penalty" there are costs associated with labor, high purity chemicals and the "wet clean station" required. There is also the chance of breaking the expensive quartzware and there are hazards involved with acid use and disposal.
In-situ plasma cleaning of LPCVD furnace tubes has numerous advantage in view of the fragility of the quartz apparatus involved. Convenience is also important since it is advantageous to perform the clean frequently to avoid heavy tube wall film buildup that will flake and cause wafer contamination. It is further desirable to use existing parts of the LPCVD equipment as far as possible to minimize added clean function expense.
Gas plasma "dry cleaning" offers these advantages provided a reliable means of introducing the plasma can be created.
At present there is only one commercially available "dry cleaning" system utilizing a gas plasma for "in situ" tube cleaning. It comprises a long electrode structure, suitably supported to permit insertion into an empty furnace tube. Appropriate pressure levels and gas flows are established and the electrodes are powered from an external source to provide a gas plasma that cleans the tube. The wafer boat and internal boat support structures can not be cleaned due to the presence of the electrodes.
The unit is bulky and interferes with the operation of the other tubes in the furnace bank while being used. "Setup" requires operator involvment. There is the possibility of quartzware contact damage and contaminant introduction from the fabric of the inserted electrode.